“I think i got it,” says Alekos Simoni with a grin, returning an electronic fly zapper called “The Executioner” to a nearby metal shelf. With a deft flick of his wrist he has done away with a genetically modified mosquito that was making a bid for freedom. There are many levels of containment to ensure such creatures do not leave this basement laboratory at Imperial College, London. But none, perhaps, quite so satisfying as The Executioner.

The extermination that the creatures in Mr Simoni’s lab are designed to take part in is less viscerally gratifying—but far more consequential. The mosquitoes are being fitted with a piece of dnacalled a gene drive. Unlike the genes introduced into run-of-the-mill genetically modified organisms, gene drives do not just sit still once inserted into a chromosome. They actively spread themselves, thereby reaching more and more of the population with each generation. If their effect is damaging, they could in principle wipe out whole species.

To engineer an extinction is quite a step. But it is not unprecedented. In 1980 Variola, the smallpox...